Packaged electronic chips that are mounted on printed circuit boards (PCBs) typically need to be tested. Frequently, prior testing was done at a wafer level after the chips have been largely fabricated, but before the chips are diced apart and packaged. Such a test is often called a wafer test and sort operation, since good chips can be sorted from bad chips that fail the test, saving time and money since the bad chips are discarded (or re-worked) before the effort of packaging the chips. Additional functional testing is often done after the chip is assembled to its first-level packaging, for example, when an integrated circuit having solder-ball connections in a ball-grid array (BGA) is attached to a multiple-layer-ceramic (MLC) flip-chip substrate (FC substrate). Such an assembly often has larger solder-ball connections for connecting to a PCB, and is called a FCBGA device. One or more such devices are mounted to a PCB to form a printed-board assembly (PBA).
There are failure modes of PBAs that are caused by or induced by differences in the respective coefficient of thermal expansion (CTE) of the various parts, e.g., of the silicon chip, the FC substrate, the PCB, and the solder-ball interconnections between various parts.
Conventional board-level test procedures sometimes include temperature cycling wherein the printed circuit board and its components are placed within a chamber that can be heated or refrigerated. To test a design's capability to withstand years of use, the temperature in the chamber is cycled from one extreme to another. Even so, some design flaws will not be discovered. Undiscovered design errors can result in a substantial capital cost to the chip and PBA manufacturer. Other testing needs include testing to verify the capabilities of new manufacturing processes (such as new solder compositions or new assembly processes) as well as manufacturing stress testing to precipitate and detect latent defects that were due to defective materials and/or manufacturing process errors.
What is needed is a fast, simple, inexpensive, reliable method and apparatus to test electronic chips and their connections to printed board assemblies, so that the tester is compact and quickly detects many temperature-dependent faults.